Take a look at https://recapamac.com.au/ . The chap is no longer in the recapping business, but offers some helpful resources. Also consider replacing the PRAM battery -- I'm not a vintage iMac expert but know that this has been an issue with other older Macs.

It has its original hard drive which still works

For reliability you should really switch to an IDE SSD.

Yes, they do exist. OWC sells them, albeit in laptop HDD size, so you will need an IDE adapter.

I have an old Mac Classic from 1990-1991 or so sitting in my basement collecting dust. I need to clean it up and crack it open to do a full recap soon. I’m going to grab a BlueSCSI to replace the internal hard drive with a fast SD card. These old machines absolutely FLY when you get them on solid state storage. It’s pretty amazing how nice they feel to use when everything loads instantly!